Document/gpu: Use new vm_fault_t type

We have introduce new return type vm_fault_t for
fault handler. Update the document for the same.

Signed-off-by: Souptick Joarder <jrdr.linux@gmail.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20180904041505.GA2712@jordon-HP-15-Notebook-PC

Documentation/gpu/drm-mm.rst

@@ -297,7 +297,7 @@ made up of several fields, the more interesting ones being:
 	struct vm_operations_struct {
 		void (*open)(struct vm_area_struct * area);
 		void (*close)(struct vm_area_struct * area);
-		int (*fault)(struct vm_fault *vmf);
+		vm_fault_t (*fault)(struct vm_fault *vmf);
 	};